The invention proceeds from a system for generating a signal for hydraulically adjusting the transmission ratio of a continuously variable transmission in a motor vehicle.
Continuously variable vehicle transmissions having a thrust band are, for example, known from DE 196 49 483 A, EP 0 451 887 A or EP 0 797 742 A. As will be explained in detail in the following, the adjustment of the transmission ratio takes place hydraulically via the adjustment of a so-called primary valve. To adjust the transmission ratio, an electric current is applied to the primary valve and thereby a specific hydraulic oil pressure is adjusted on the primary end, that is, on the drive end of the transmission. The primary valve can, for example, be configured as a magnetic valve. In DE 196 49 483 A, the suggestion is made to limit the adjustment of the transmission ratio in dependence upon the detected rotational movement of the primary end.
The tension of the belt can be adjusted with the aid of a pressure limiting valve at the secondary end, that is, at the output end of the transmission.
For a rapid transmission ratio adjustment of the transmission, for example, during a high deceleration of the vehicle, a large amount of oil is needed within a short time for the adjustment of the transmission ratio. The pressure in the secondary oil chamber can collapse when the hydraulic oil pump does not make available an adequate oil volume flow because the primary valve has adjusted an opening cross section which is too large. As a consequence, the belt tension can become too low and the thrust belt slips. This is intended to be prevented by the present invention in accordance with the task.
As already mentioned, the invention proceeds from a system for generating a signal to hydraulically adjust the transmission ratio of a continuously variable transmission in a motor vehicle. A valve is provided which influences the hydraulic connection from an pressure reservoir, which has a hydraulic pressure, to a pressure chamber. The adjustment of the transmission ratio takes place via the drive of at least the valve with the electrical signal. For this purpose, the electrical signal is generated in such a manner that it is limited to maximum and/or minimum values.
The essence of the invention is that the maximum and/or minimum values are determined in dependence upon the hydraulic pressure.
With the limiting in accordance with the invention, it is ensured that an adequate pressure reserve is made available for the adjustment of the belt tension notwithstanding the adjustment of the transmission ratio. A collapse of the secondary pressure, which is necessary for the belt tension, and the problem of slippage of the belt element is thereby avoided. The invention permits a rapid transmission ratio adjustment because the speed of adjustment goes to the limits of the maximum possible adjustment with the view toward a reliable belt tension adjustment.
In an advantageous configuration, it is provided that the adjustment of the pregivable tension takes place by the adjustment of the hydraulic pressure. This pressure can be detected by a hydraulic sensor.
The adjustment of the belt tension takes place especially in that the hydraulic pressure is adjusted (especially controlled) to a desired value. The limits of the invention are then selected in such a manner that the hydraulic pressure can be adjusted to the desired value in each case.
In a first variation of the invention, it is provided that the maximum and/or minimum values are determined in dependence upon a comparison or the difference between the hydraulic pressure and the corresponding desired value. The maximum and/or minimum values (I max, I min) can be determined in such a manner that the hydraulic connection from the pressure reservoir, which exhibits the hydraulic pressure (Ps,act), to the pressure chamber 12 is then limited when the desired value (Ps,des) exceeds the hydraulic pressure (Ps,act) by a pregivable extent.
A second variation of the invention is directed to the hydraulic pump for pumping the hydraulic volume flow with the hydraulic pump being driven especially by the vehicle motor. Here, it is provided that:
a pumped volume value, which represents the instantaneous hydraulic volume flow, is determined at least in dependence upon the operating state of the hydraulic pump and especially in dependence upon the rpm of the vehicle motor; and,
a hydraulic volume flow, which is required for the hydraulic oil pressure supply, is determined at least in dependence upon the desired value; and,
the maximum and/or minimum values are determined in dependence upon a comparison or the difference between the pumped volume value and the required hydraulic volume flow.
In this variation of the invention, it can be provided that the determination of the required hydraulic volume flow continues to be performed in dependence upon a comparison or upon the difference between the hydraulic pressure and the desired value (Ps,des). The determination of the required hydraulic volume flow can take place by means of a characteristic field. The characteristic field is adapted to tolerances of the transmission, for example, the manufacturing-caused tolerances and/or deterioration-caused tolerances by the comparison or by the difference between the hydraulic pressure and the desired value. The invention thereby permits a rapid transmission ratio adjustment because the speed of adjustment goes to the limits of the maximum possible adjustment because manufacturing and deterioration tolerances need not be considered in the context of a worst case scenario.